1. Field of the Invention
The invention relates in general to a mixer, and more particularly to a mixer capable of providing a lower operation voltage.
2. Description of the Related Art
A mixer converts a frequency of an input signal into another frequency and may be used in, for example, a down-sampling procedure or an up-sampling procedure. FIG. 1 (Prior Art) is a circuit diagram showing a conventional mixer 100. Referring to FIG. 1, the mixer 100 includes transistors Q1, Q2, Q3, Q4, Q5 and Q6 and a current source Cs1. The mixer 100 must have a higher operation voltage in order to have the high gain, the good linearity and the low noise.
With the progress of the CMOS manufacturing technology and the coming of the system-on-chip (SOC) age, the operation voltage of the analog circuit also has to be reduced.
In the mixer 100 of FIG. 1, when the provided operation voltage is decreased, voltages at the drains of the transistors Q6 and Q3 are also decreased so that the transistors Q3 and Q6 cannot work in the saturation region and the overall behavior of the circuit is influenced. In order to reduce the voltage drop, the current source Cs1 is usually removed, but this method increases a common mode gain.